At Higher Tier, you are expected to write and balance half equations for all four possible products in aqueous electrolysis:
Cathode (reduction — gain of electrons):
- Hydrogen:
2H⁺ + 2e⁻ → H₂ - Copper:
Cu²⁺ + 2e⁻ → Cu
Anode (oxidation — loss of electrons):
- Chlorine:
2Cl⁻ → Cl₂ + 2e⁻ - Oxygen:
4OH⁻ → O₂ + 2H₂O + 4e⁻
The oxygen half equation is the most complex — notice that 4 OH⁻ ions produce 1 O₂ molecule and 2 H₂O molecules. Check: 4 oxygens on each side ✓, 4 hydrogens on each side ✓, 4 negative charges = 4 electrons lost ✓.
This higher tier covers Higher Tier: Half Equations for Aqueous Electrolysis within Electrolysis of Aqueous Solutions for GCSE Chemistry. Revise Electrolysis of Aqueous Solutions in Electrolysis for GCSE Chemistry with 21 exam-style questions and 15 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 7 of 13 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Practice questions for Electrolysis of Aqueous Solutions
When sodium chloride (NaCl) is dissolved in water, which four types of ion are present in the solution?
Describe the three products formed when concentrated brine is electrolysed, and state where each is produced.